Some current image capturing devices that have a solid-state image sensor such as a CCD or CMOS sensor have a live view function. A live view function is a function that allows confirmation of an object image by sequentially outputting an image signal that is continuously read out from an image sensor onto a display device such as a liquid crystal display or the like arranged on the rear surface or elsewhere on a camera. Also, as a common method of using a light beam that passes through an imaging lens in an auto focus detection/adjustment method of an image capturing device, there is a contrast detection method (called a blur method) and a phase difference detection method (called a deviance method).
The contrast detection method is a method that is often used in a moving image capturing video movie device (camcorder) or digital still camera, wherein an image sensor is used as a focus detection sensor. In this method, an output signal of an image sensor, particularly information of a high-frequency component (contrast information) is focused on, and an imaging lens position for which that evaluation value becomes largest is set as a focus position. However, because an evaluation value is determined while moving the imaging lens in small amounts, and because it is necessary to move the lens until an evaluation value is determined with certainty to be the maximum, this method is not suitable for high-speed focus adjustment operation, and is also called a mountain-climbing method.
The phase difference method, on the other hand, is often used in single-lens reflex cameras that use silver halide film, and is the technique that contributes most to the practical use of the auto focus detection (Auto Focus: AF) single-lens reflex camera. In the phase difference method, a light beam that passes through a projection pupil of an imaging lens is split into two, and each of the split light beams are received by a pair of focus detection sensors. The amount of deviation in the focus direction of the imaging lens is then directly determined by detecting the amount of deviation in the signals output based on the amount of received light, that is, the amount of relative positional deviation in the direction the light beam was split. Therefore, the amount and direction of focus deviation can be obtained by executing an integration operation once by the focus detection sensors, and a high-speed focus adjustment operation becomes possible. However, in order to split the light beam that has passed through the projection pupil of the imaging lens into two and obtain a signal corresponding to each light beam, it is common to provide a light path splitting means such as a quick-return mirror or half-mirror in the imaging light path, and to provide a focus detection optical system and AF sensor in front. For this reason, there is the disadvantage that the apparatus becomes large and expensive. There is also the disadvantage that operation cannot be done during live view since the quick-return mirror is retreated.
In order to eliminate the aforementioned disadvantages, a technique to provide an image sensor with a phase difference detection function and to realize high-speed phase difference AF without the need for a dedicated AF sensor has been proposed. For example, in Japanese Patent Laid-Open No. 2000-156823, a pupil splitting function is provided to a portion of photoreceptors (pixels) of an image sensor by de-centering a sensitivity region of a photoreceiving unit from the light axis of an on-chip microlens. Phase difference focus detection is then executed by arranging these pixels as focus detection pixels at predetermined intervals between a group of imaging pixels. Because the locations at which the focus detection pixels are arranged correspond to defective portions of the imaging pixels, image information is created by interpolation from surrounding imaging pixel information.
Furthermore, in Japanese Patent Laid-Open No. 2000-292686, a pupil splitting function is provided by splitting a photoreceiving unit of a portion of pixels of an image sensor into two. Phase difference focus detection is then executed by arranging these pixels as focus detection pixels at predetermined intervals between a group of imaging pixels. In this technique as well, because the locations at which the focus detection pixels are arranged correspond to defective portions of the imaging pixels, image information is created by interpolation from surrounding imaging pixel information.
However, because image information is created by interpolation from surrounding imaging pixels at locations at which focus detection pixels are arranged and that are defective portions of the imaging pixels in the aforementioned Japanese Patent Laid-Open No. 2000-156823 and Japanese Patent Laid-Open No. 2000-292686, there are cases in which interpolation cannot be done properly, depending on the object. For this reason, although the deterioration in image quality is small in the case that the number of focus detection pixels is small compared to the number of normal imaging pixels, there is the problem that the deterioration in image quality becomes large with the increase in the proportion of focus detection pixels.
Also, in the case that live view is executed, it is necessary to perform read out from the image sensor at high speed in order to realize a target frame rate, and a method of thinning out a portion of the pixels of the image sensor and performing readout at high speed in order to achieve this is known. However, when pixel arrangement is done such that focus detection pixels are included in the read out pixel signal, the proportion of focus detection pixels with respect to imaging pixels increases in comparison to a case in which all pixels are read, and the effect on image quality becomes large.
With respect to the aforementioned problem, the following proposal is given by Japanese Patent Laid-Open No. 2000-156823. That is, when thinning out and reading out, live view is executed in a state in which the focus detection pixels are not read out, and when auto focus adjustment is executed, live view is terminated, and the focus detection pixels are read out. However, Japanese Patent Laid-Open No. 2000-156823 has the problem that because the focus detection pixels are not read out when a portion of the pixels of the image sensor are thinned out and read out at high speed, auto focus adjustment cannot be executed while executing live view.